A patent application of Fauver et al. published as US-2004-0076319-A1 on Apr. 22, 2004, incorporated herein by this reference, discloses a method and apparatus for continuously scanning the focal plane of an optical imaging system along an axis perpendicular to said plane through the thickness of a specimen during a single detector exposure.
One such method is accomplished by moving an objective lens, thereby scanning the focal plane through the thickness of the specimen region, such that the entire specimen thickness is scanned continuously during a single detector exposure interval. A pseudoprojection image is thereby generated whose resolution can depend on the depth of focus of a moving focal plane, as well as on the lateral spatial resolution (i.e., the resolution within the focal plane). The procedure is repeated from several perspectives over an arc of up to 180 degrees, using one or more pairs of light sources and detector arrays simultaneously. The specimen can be rotated and/or translated to acquire additional viewpoints. In this way, a set of pseudoprojections is generated, which can be input to a tomographic image reconstruction algorithm, such as filtered backprojection, to generate a three-dimensional image.
Known techniques work well for a specimen that is positioned in the center of a rotating capillary tube because the specimen will not move out of an initial focal plane during rotation. However, many specimens are positioned off center and will translate out of an initial focal plane. Such offset positions can cause focusing errors and adversely affect post-imaging acquisition reconstruction of the specimen.